Mid Atlantic to me is someone UK attempting to sound American, Joe

Or vice versa, Jon...

Or even (as in my own case) someone who has grown up in one place and lived quite a while in the other.

I once entered a trad singing competition at a folk festival in the UK. Amongst the comments I received from the judges was a criticism for singing my song in a weird sort of mid-Atlantic accent. I remember wondering (since I did not speak much before or after singing) whether the judges had realised I'm a transplanted Yank and was honestly singing in "my own voice". Guess I'll never know...